Acute Insecticides

Not surprisingly, some of the best known botanical insecticides and those with the longest use history are those that are truly insecticidal, i.e., that kill insects shortly after contact most extensively used botanical to date remains pyrethrum, extracted from the dried flower heads of Tanacetum cinerariaefolium (Asteraceae). As classical axonic neurotoxins, the pyrethrins and naturally occurring analogues (see Figure 7.1) are famous for their fast knockdown effect on flying insects. Indeed it was only the rapid photolability of these compounds that limited their broader use in agriculture, and thus stability of the molecules became the key goal for development of the synthetic pyrethroid insecticides (see Chapter 3). Another long used botanical is nicotine (from foliage of Nicotiana spp. and related Anabasis spp. [Solanaceae]) (see Figure 7.1). Nicotine, an alkaloid, is a synaptic neurotoxin that served as the progenitor of the now widely used neonicotinoid synthetic insecticides (see Chapter 4). As a botanical it has fallen out of favour in recent years owing to the extreme risk it poses to humans.

Figure 7.1 Structures of the major constituents of botanical insecticides in current use. (See Table 7.1 for details).

Several other botanicals that have seen some commercial use are insecticidal -in that they kill insects after a single exposure - but with less rapid action. Among these are rotenone (see Figure 7.1), an isoflavonoid from the rhizomes of Derris species (Fabaceae) that is a cytotoxin targeting energy production in mitochondria. Rotenone enjoys some minor use as a consumer insecticide, but the major use of this material is a commercial fish poison. Sabadilla refers to the seed powder of the South American lily Schoenocaulon officinale (Liliaceae) and consists of a mixture of ceveratrum alkaloids with a neurotoxic mode of action not unlike that of the pyrethrins. Ryanodine (see Figure 7.1), an alkaloid found in the wood of the South American shrub Ryania speciosa (Flacourtiaceae), has been used to a limited extent, primarily in organic production. It has a mode of action based on poisoning of neuromuscular junctions. Lastly, quassin (see Figure 7.1) also merits mention as a botanical insecticide. It was originally obtained from the wood and bark of the Central American tree Quassia amara (Simaroubaceae), and now is also obtained from closely related shrubs in the genera Aeschrion and Picrasma. Like many other botanicals, quassin fell out of favour in the 1950s with the advent of synthetic insecticides, but one company in Germany is reintroducing it to the market as an alternative product for use in organic food production.15

Leading a resurgence in interest in botanical insecticides were products based on seed extracts and oils from the Indian neem tree, Azadirachta indica (Meliaceae), and the outstanding insect antifeedant/insect growth regulator, azadirachtin (see Figure 7.2).16 Though known for centuries in India, the biological activities of this substance were not investigated and documented in the international scientific literature until the 1970s, and the first neem-based insecticide was introduced into the US market around 1990. At present, refined neem-based products are used in the US, Europe and India; crude extracts are used in Africa, Latin America and throughout Asia. Azadirachtin works primarily through interference with molting by blocking the synthesis and release of ecdysteroids in insects, but numerous other neuroendocrine effects have been reported. Owing to these modes of action, azadirachtin/neem products take considerable time to kill exposed insects, although many phytophagous pests suspend feeding shortly after exposure, providing the desired crop protection effect. The relatively high cost of refined neem products appears to have been a major impediment to their success in North America and Europe.17

The other group of botanicals that have rekindled interest in this category are insecticides based on plant essential oils. The exempt status of certain oils used as flavouring agents in foods and beverages (viz. those obtained from cinnamon, cloves, mints, rosemary and thyme) from registration as pesticide active ingredients in the USA facilitated the development and commercialization of insecticides based on these oils a decade ago.18 Monoterpenoids and sesqui-terpenoids, major constituents of the oils, are neurotoxic to a wide range of o

Azadirachtin A: R1 = Ac; R2 = Tigloyl; R3 = OH Azadirachtin B: R1 = Tigloyl ; R2 = H; R3 = H

Salannin

Figure 7.2 Structures of azadirachtin A and B, and salannin, major constituents of neem seeds (Azadirachta indica) and botanical insecticides based on neem seed extracts.

Azadirachtin A: R1 = Ac; R2 = Tigloyl; R3 = OH Azadirachtin B: R1 = Tigloyl ; R2 = H; R3 = H

Salannin

Figure 7.2 Structures of azadirachtin A and B, and salannin, major constituents of neem seeds (Azadirachta indica) and botanical insecticides based on neem seed extracts.

insects and mites. While their effects on the neuromodulator octopamine have been demonstrated in some insects, there is emerging evidence that certain of the terpenoids may target other sites in the insect nervous system.19 They can be very fast acting, even faster than some conventional neurotoxic insecticides, albeit at doses orders of magnitude greater than conventional products. Nonetheless their relative safety to humans and other nontarget species, short environmental persistence, and the public's general familiarity with them (as flavourings, fragrances and in aromatherapy) makes them ideally suited for consumer uses in and around the home and on companion animals.

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